Because of recent developments in power semiconductor devices such as power MOSFETs and insulated gate thyristors (IGTs), electronically commutated motors such as variable reluctance (VR) and brushless permanent magnet (PM) motors have gained attention relative to other types of motors suitable for variable-speed drive applications. This increased attention derives from the fact that electronically commutated motors compare very favorably with other types of motors typically used as variable-speed drives. For example, their speed versus average torque curves are fairly linear with no discontinuities. They are rugged and robust and therefore well suited for heavy duty use. They have excellent heat dissipation qualities, and they do not require brushes or slip rings. Moreover, using state-of-the-art semiconductor technology for controllers, the efficiency of brushless commutated motors compares very favorably with other classes of variable-speed motors such as invertor-driven AC motors. Additionally, VR motors are the lowest cost type of motor to manufacture. Their drive circuits are the simplest and lowest cost compared to drives for other variable-speed motors.
As variable-speed drives, VR motors are designed for efficient power conversion rather than for particular torque or control characteristics typically required in stepper motor applications, and the pole geometry and control strategies differ accordingly. For example, the number of rotor teeth is relatively small in an electronically commutated reluctance motor (e.g., variable reluctance stepper motors), giving a large step angle, and the conduction angle is, generally, modulated as a function of both speed and torque to optimize operation as a variable-speed drive. In continuously rotating, variable speed applications, VR motors are often called switched reluctance or SR motors to distinguish them as a class from VR motors operated as stepper motors. Hereinafter, continuous drive VR motors are simply called SR motors.
Electronically commutated motors conventionally have multiple poles on both the stator and rotor--i.e., they are doubly salient. For the SR motor, there are phase windings on the stator but no windings or magnets on the rotor. For PM motors, however, permanent magnets are mounted on the rotor. In a conventional configuration of either type of motor, each pair of diametrically opposite stator poles carry series connected windings that form an independent phase of a power signal.
Torque is produced by switching current into each winding of a phase in a predetermined sequence that is synchronized with the angular position of the rotor, so that the associated stator pole is polarized and the resulting magnetic force attracts the nearest rotor pole. The current is switched off in each phase before the poles of the rotor nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of the attraction would produce a negative or breaking torque. For SR motors, the torque developed is independent of the direction of current flow in the phase windings so that unidirectional current pulses synchronized with rotor movement can be applied to the stator phase windings by a convertor using unidirectional current switching elements such as thyristors or transistors.
The converters for electronically commutated motors operate by switching the stator phase current on and off in synchronism with rotor position. By properly positioning the firing pulses relative to the angle of the rotor, forward or reverse operation and motoring or generating operation can be obtained.
Usually, the desired commutation of a phase current is achieved by feeding back a rotor position signal to a controller from a shaft position sensor--e.g., an encoder or resolver. For cost reasons in small drives and reliability reasons in larger drives and to reduce, weight and inertia in all such drives, it is desirable to eliminate this shaft position sensor.
To this end, various approaches have previously been proposed for indirect sensing of the rotor position by monitoring terminal voltages and currents of the motor. One such approach, referred to as waveform detection, depends upon the back electromotive forces (emf) and is, therefore, unreliable at low speeds. Another approach is described in U.S. Pat. Nos. 4,611,157 and 4,642,543 assigned to General Electric Company of Schenectady, N.Y. In these patents, the average d.c. link current is used to dynamically stabilize a drive for a SR motor. Such systems are believed to be limited by the average nature of their feedback information and by the tendency of the SR motor to jitter at start-up.
In U.S. Pat. No. 4,772,839 assigned to General Electric Company of Schenectady, N.Y., a sampling pulse is injected into each of the unenergized phases of a SR motor. Rotor position is estimated by firing an unenergized phase for a time period short enough that the build-up of current and the motion of the rotor are negligible. The slope of the initial current rise in the unenergized phase is used to determine inductance. By sampling more than one phase, the direction of the rotor rotation is determined. Specifically, the sampling in each phase provides two possible angles for the rotor. Two angles are possible because the rotor can be turning in either clockwise or counterclockwise directions. By sampling in two phases and comparing the rotor angles derived from the sampling of the two phases, the correct or actual rotor angle is identified since only one of the two angles identified by each phase will be equal to one of the two angles in the other phase. This common angle is identified as the actual position of the rotor with respect to its direction of rotation. The estimated rotor angles are derived from absolute values of the inductance. Therefore, the control system must be precisely matched with a particular motor.
In U.S. Pat. No. 4,520,302, assigned to National Research Development Corporation, a control circuit for SR motors is disclosed that utilizes the fact that the inductance of a phase winding is dependent on rotor position and varies substantially sinusoidally from a maximum to a minimum as the rotor advances over a pole pitch. The control circuit utilizes the variation of inductance to measure certain characteristics of current flow in an appropriate one of the windings in order to derive an indication of rotor position and thus provide closed-loop control of the motor. Like the control system of the foregoing '839 patent, the absolute value of the inductance of the winding must be determined in order to derive a rotor position.